Authors: Indrek Must, Geoffrey M. Spinks, Alvo Aabloo,

Publish Date: 2016

Volume:, Issue:, Pages: 1-16

Abstract

Carbon is a distinctive electrode material for actuators, as it is available in a wide variety of forms, ranging from monoliths to powders, fibers, and yarns. The diversity in the properties of different carbonaceous materials is also expressed in a variety of actuation mechanisms. This chapter considers two classes of actuators – electrochemically and electrothermally driven actuators – which both make use of carbonaceous materials as active elements. In both of the listed types of actuators, carbon is especially advantageous because of its chemical and thermal inertness and also because of its high intrinsic electrical conductivity. The working principles of different actuators, having carbonaceous electrodes, are drastically different and so are the optimization criteria for selecting a particular type of carbon for a particular type of actuator. This chapter is to explain some important practical considerations for successful experimentation with the carbon-based actuators. Special attention is bestowed on the choice of materials and the choice of appropriate electrical driving signal. The effects caused by the ambient environment are discussed. Finally, a selection of commonly used characterization methods is suggested.When starting experimentation with any type of actuator, their particularly distinctive characteristics, and possible multifunctional nature, should be considered, as they can be essential in consideration of possible applications and characterization methods for the particular types of actuators.In the electrothermally driven actuators, synergistic effect between a carbon filament as the host material and a polymeric filler, having large thermal expansion coefficient, is achieved. These systems have only recently been described (Lima et al. 2012) and are based on electrochemically driven carbon nanotube yarn torsional actuators (Foroughi et al. 2011). The electrothermal systems also utilize carbon nanotube yarns but incorporate a guest material as filler. Electric heating causes guest volume expansion that then produces a combination of yarn torsion and lengthwise contraction.Whereas electrothermally driven actuators, although more straightforward in their construction, have gained their popularity primarily during the past few years’ time, the electrochemically driven actuators, having carbonaceous electrodes, have been first proposed in 1999 (Baughman et al. 1999). The working principle of the electrochemically driven actuators involves many concurrent mechanisms, the ratio of which is still under dispute to date. It is generally accepted that the strain difference between the oppositely charged electrodes is related to the shift in the local concentration of the electrolyte ions that accompanies with the buildup of electric double-layer (Kosidlo et al. 2013).The introduction of carbonaceous electrode materials for the electrochemically driven actuators intersected two classes of electrochemical devices that were considered as being separate beforehand – the electromechanical actuators and the electric double-layer capacitors (also known as supercapacitors). Although the conventional supercapacitors and the “conventional” electrochemically driven actuators have notable differences in their particular optimization parameters, their governing physical processes are virtually identical.The similarity between the carbonaceous, electrochemically driven actuators, and the supercapacitors is particularly noteworthy. Among the wide range of existing types of “smart” actuators, whose working principle involves electro-osmosis of ions in the extent of a layered microporous laminate, the actuators with carbonaceous electrodes resemble the most to the supercapacitors. The high level of similarity is anticipated, as identical carbonaceous materials can be used in construction of either of the two devices – a supercapacitor (Simon et al. 2013) or an actuator (Kosidlo et al. 2013). After the introduction of carbon as the superior active material for the electrochemically activated actuators in 1999, the emerged types of actuators resembled to a great extent to the supercapacitors, which had been in commercial production for many decades already – since 1978 (Miller 2007). It has been demonstrated that (a) the electrochemically driven actuators have a decent energy-storage capability (Torop et al. 2009, 2011) and (b) opportune employment of the capacitive nature of the electrochemically driven actuators gives completely new functionalities to these laminates, originally intended for use as actuators (Must et al. 2013). In order to emphasize the capacitive nature of the electrochemically driven actuators, “actuating” and “charging” of this type of actuators are used as synonyms in this chapter.The capacitive nature of the electrochemically driven actuators with carbonaceous electrodes is of particular importance to consider when starting experimentation with this type of actuators – the capacitive properties of the actuators determine the most suitable driving waveform and characterization methods.The electrothermally driven actuators benefit the most from carbons that have directional nanostructures, resulting in high mechanical yield strength, but also in sufficient electrical conductivity along the nanostructure direction. Twisted yarns of multiwalled carbon nanotubes are the most extensively studied of these relatively new actuator materials. Twisting the multiwalled CNTs adds considerable strength (Zhang et al. 2004), but the method of preparation also allows the ready addition of guest materials incorporated within the continuously twisted CNT bundles (Lima et al. 2011). Electrothermally activated hybrid CNT yarns have been produced by infiltrating twisted CNT yarns with molten paraffin wax. The yarns were prepared by drawing from a “forest” of CNTs produced by chemical vapor deposition. The drawn CNT sheet consists of interconnected and oriented multiwalled CNTs. Twist insertion produces the twisted yarn that is stable. “Overtwisting” generates a coiled yarn structure that is generally not stable when released. Dipping the yarn into molten wax fills the pore space with paraffin. Solidification of the wax by cooling can fix the coiled structure when released. Other guest materials have also been incorporated into the twisted and coiled CNT yarns, including synthetic rubbers (Chun et al. 2014). Other host materials have also been recently reported like niobium nanowire yarns (Mirvakili et al. 2013).In the selection of carbons for use as the electrode material in electrochemically driven actuators, the primary factor of choice is the specific surface area of the carbons. Generally, an actuator that has higher capacitance also has higher actuation performance; however, this principle is valid in the extent of the same electrode material only (Palmre et al. 2009).